Visco-elasto-plastic analysis of steel frames in fire

A finite element formulation for the analysis of two-dimensional steel frames either at ambient or elevated temperature is presented. The formulation includes both geometric and material nonlinearities, and a creep model. A new feature is the manner in which strain reversal is incorporated into the program. The program can analyze steel frames subjected either to (1) increasing external loads at ambient temperature or, (2) constant external loads at elevated temperatures. The program then gives predictions of (1) a collapse load or (2) critical temperature, whichever the case may be. This program has been applied to a number of experimental and analytical benchmark tests. Excellent agreement has been obtained in all these cases. Following on these benchmark tests, creep effects on the buckling of heated columns have also been studied. It shows that creep generally starts to be dominant beyond 400°C. In connection to this, the current simplified method for calculating the critical temperature in Eurocode 3 Pt. 1.2 is shown to be unsafe for columns with intermediate slenderness ratio under both uniform heating and nonuniform heating.